1. Field of the Invention
The present invention relates to high-temperature seals and their characteristics. More particularly, the invention concerns a technique for in-sit-up monitoring of seal integrity.
2. Description of Prior Art
By way of background, various high-temperature device applications, such as solid oxide fuel cells, involve structures composed of multiple ceramic layers, or interleaved layers of ceramic and metal. To eliminate the possibility of gases (such as fuel constituents) prematurely mixing or leaking from interior regions of the structure, the ceramic-to-ceramic or ceramic-to-metal interfaces of adjacent layer pairs are typically joined and sealed together with a glass or ceramic seal. These seals must be capable of withstanding the high-temperature operating environment of the device over its operational lifetime. The seal materials must also be carefully matched with the adjacent layers in order to minimize, to the extent possible, differences in thermal expansion coefficients, thermal shock resistance, fracture toughness, and mechanical strength characteristics. Failure to do so can lead to the development of severe thermal stresses and consequent seal failure during various device operational modes.
Unfortunately, it is very difficult to monitor seal integrity once a high-temperature device has been fabricated. Is likewise very difficult to determine the cause of a seal failure when such failure occurs. Quite often, the failure is so catastrophic that it destroys the ability to evaluate the reason for the failure. Similarly, any attempt to disassemble the failed device may destroy the evidence required for failure evaluation. As a consequence, the conventional approach used when designing and manufacturing seals for high-temperature applications is to develop mathematical models for calculating the seal stresses that are likely to arise during device operation, and thereby attempt to predict operational regions of safety or failure.
It is to improvements in seal integrity monitoring that the present invention is directed. In particular, the invention addresses the need for an in-sit-up evaluation technique whereby the integrity of high-temperature seals can be monitored during device fabrication and also thereafter during field operation.